<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<script>
			let arr=[30,40,10,5,3,7,8]
			// let arr1=arr.filter(function(a){
			// 	return a
			// })
			let index=arr.findIndex((value)=>{
				value==40
			})
			console.log(index)
			//
		</script>
	</body>
</html>